Screening of bimetallic heterogeneous nanoparticle catalysts for arene hydrogenation activity under ambient conditions

Article abstract of DOI:10.1021/ic901928j

This study focuses on the application of a simple screening approach to prepare and test heterogeneous mono- and bimetallic nanoparticle (NP) catalysts for arene hydrogenation activity under ambient conditions in a quick and time efficient manner, as well

Full text of DOI:10.1021/ic901928j

2706 Inorg. Chem. 2010, 49, 2706–2714
DOI: 10.1021/ic901928j
Screening of Bimetallic Heterogeneous Nanoparticle Catalysts for Arene
Hydrogenation Activity under Ambient Conditions
Nicole A. Dehm, Xiaojiang Zhang, and Jillian M. Buriak*
Department of Chemistry, University of Alberta and the National Institute for Nanotechnology (NINT),
Edmonton, Alberta, Canada T6G 2G2
Received September 30, 2009
This study focuses on the application of a simple screening approach to prepare and test heterogeneous mono- and
bimetallic nanoparticle (NP) catalysts for arene hydrogenation activity under ambient conditions in a quick and time
efficient manner, as well as detailed testing and characterization of identified active catalysts. Over 90 mono- and
bimetallic NP catalysts supported on alumina were efficiently screened for arene hydrogenation activity under ambient
conditions using toluene as a model substrate. Through this approach, four catalysts were determined to be active:
RhPt/Al2O3, RuPt/Al2O3, IrPt/Al2O3, and IrRh/Al2O3. These catalysts were further synthesized and tested in bulk, and
RhPt/Al2O3 was confirmed to be the catalyst with the highest observed rate of all the bimetallic combinations screened.
Further studies were then performed, and the metal loading, temperature, pressure, and substrate to metal ratios were
varied to determine the effects of these variables on the activity of the RhPt/Al2O3 catalyst and a CS2 poisoning study
was performed on this catalyst to determine the number of active sites. From the temperature studies, the activation
energy was calculated to be 30.4 kJ/mol, which is moderate when compared to other arene hydrogenation catalysts
(42.0 kJ/mol for the hydrogenation of toluene with Ru NPs,1 and 34.7 and 45.6 kJ/mol for benzene hydrogenation with
cuboctahedral and cubic Pt NPs, respectively,2 have been reported). Transmission electron microscopy (TEM), X-ray
photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) surface area measurements were used to
characterize the active catalysts, where it was observed that very small zero oxidation state metal NPs were well
dispersed throughout the high-surface area alumina support.
Introduction
challenging catalytic reaction is of great importance to the
petrochemical industry.4-7 This reaction is, however, diffi-
cult because of the stability of the aromatic rings, and thus
harsh conditions are requiredinexcess of 340°C and 6.9 MPa
to achieve successful hydrogenation.7 Complicating matters,
metal catalysts are vulnerable to poisoning because of the
sulfur and nitrogen contaminants found in bitumen.8 All
these factors increase crude oil costs and thus it is of interest
to find new families of catalysts that can function with less
energy input, are sulfur and nitrogen tolerant, and are
selective with respect to the desired hydrogenation products.
Nanoparticle (NP) catalysts have been used for a wide
variety of reactions including both olefin9,10 and arene1,2,7,8,10-62
In northern Alberta, vast areas of land contain oil sands
deposits, of which bitumen is one of the main components.
There are over 173 billion barrels of recoverable reserves
represented by the verified deposits in Alberta, which cover
approximately 140,000 km2 of land.3 Bitumen is composed of
asphaltenes, complex arenes, cycloparaffins, and other het-
eroatom containing compounds of high molecular weights,
all of which must be removed before most direct uses.4
Specifically, hydrogenation of the large polyaromatic hydro-
carbons found in bitumen is one of the steps involved in
upgrading bitumen into synthetic crude oil, and thus this
*To whom correspondence should be addressed. E-mail: jburiak@
ualberta.ca.
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Published on Web 02/16/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2707
hydrogenations. NP catalysts are advantageous for many
reasons including high surface areas and energies, unique
electronic effects, and potentially lower cost: high surface to
volume ratios mean less metal is “wasted” in the particle
interior, and higher selectivity produces fewer undesir-
able side products.63,64 Both mono-1,2,11-50,65-74 and
bimetallic7,8,12,13,51-55,60 NP catalysts have been studied for
arene hydrogenation, and bimetallic catalysts are of particu-
lar interest because of the potential for enhanced activity and
an increased tolerance to sulfur and nitrogen contaminants.8
While both unsupported and supported NP catalysts have
demonstrated activity for arene hydrogenation, supported
catalysts are more robust and also allow for easier separation
of the catalyst from the reaction mixture enabling the catalyst
to be easily recycled.37 In addition, there are important
synergistic effects between the NP catalyst and the underlying
support. For example, the acidity of the support, electron
transfer to and from the support, and epitaxial stress at the
NP-support boundary can all have a significant influence on
the activity of the catalyst.63,71,75 NP catalysts have been
supported on a variety of materials including carbon
nanotubes11-13,55 and metal oxides,7,8,14-26,51-54,60,65-71
and the latter is a very common family of supports currently
used for bitumen processing catalysts, among many others.
Metal oxides such as Al2O3, TiO2, and SiO2 are inexpensive,
widely available commercially, and allow for control of
parameters such as substrate acidity and porosity, among
others, permitting tuning of catalyst activity.71,75
At present, a limited number of bimetallic combinations
are known to be active as NP arene hydrogenation cata-
lysts.7,8,12,13,51-55,60 It would, therefore be interesting to
screen a large number of bimetallic combinations, in parallel,
to identify new bimetallic catalysts active for arene hydro-
genation under mild conditions (1 atm H2, 22 °C). Taking
into account the many possible variables, including the ratio
of the two metals, metal loading, support types, substrates,
and so forth, it would be incredibly time-consuming to test
each individual catalyst one by one in an empirical fashion.
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2708 Inorganic Chemistry, Vol. 49, No. 6, 2010
Dehm et al.
proven to be an efficient means of synthesizing and screening
large numbers of potential materials for a desired property,
leading to much shorter discovery times.76-89 For example,
Maier used IR thermography to simultaneously screen 37
potential catalysts for 1-hexyne hydrogenation and isooctane
oxidation activity in the gas phase.81 More recently Parkin-
son used an ink jet printer to print overlapping oxide
precursors onto conductive glass substrates. These materials
were then screened for photoelectrolysis activity, and a
potential catalyst was identified.78 On the basis of the prior
success and potential of the combinatorial approach for
catalyst synthesis and testing, a library of potential catalysts
could be prepared and tested for arene hydrogenation acti-
vity under ambient conditions, allowing for efficient identi-
fication of active catalysts.
In the present study, a variety of mono- and bimetallic NP
catalysts supported on alumina were synthesized and as-
sessed using a simple parallel screening approach, under
ambient conditions, for toluene hydrogenation activity. In
total, 91 catalysts were screened using 13 representative
transition metals from the first, second, and third rows of
the periodic table. One particularly active catalyst was
identified and further characterized in bulk and studied
(kinetics and materials characterizations) and is described
here.
Fischer Scientific. Carbon disulfide was purchased from
Sigma-Aldrich and was used without further purification. Iso-
propanol was dried over molecular sieves and stored under
argon before use. Toluene was purified through a solvent
purification system and was stored under argon until used.
Hydrogen, argon, and 5% hydrogen/95% argon were supplied
by Praxair.
Instrumentation. A Varian CP-3800 Gas Chromatograph
with a CP-4800 autosampler with a fused silica capillary column
and a FID detector was used to analyze the samples of the
reaction mixture. A Corning Model PC-420 Laboratory stirrer/
hot plate was used throughout, except for the temperature
studies, in which an IKA Works Ceramag Midi stirrer/hot plate
with an IKA ETS-D4 temperature sensor was used for tempera-
tures above ambient. An Agilent 6890 gas chromatography with
a 5973 mass detector was used for the GC-MS experiments. A
Parr pressure vessel, model 4774-T-SS-3000 and a model 4838
controller with a pressure display module were used for the
pressure and CS2 poisoning studies. For the transmission elec-
tron microscopy (TEM) analysis, a JEOL JEM-2200FS was
used in STEM mode. For X-ray photoelectron spectroscopy
(XPS) a Kratos Analytical, Axis-Ultra instrument was used for
the sample analysis. XPS were performed under UHV condi-
tions (<10-8 Torr). Surface areas were measured by nitrogen
adsorption at 77.3 K using an Autosorb-1 high performance
surface area and pore size analyzer (Quantachrome Instru-
ments).
Catalyst Screening Synthesis of 1 mol % Metal Loading. The
procedure for the catalyst synthesis was based on modified
procedures established by Maier.81,90 0.133 M solutions of the
required metal salts in ethanol were prepared, as was a 1.788 M
solution of aluminum-sec-butoxide in dichloromethane and a
solution consisting of 1.0 mL of ethanol, 13 μL of concentrated
hydrochloric acid, and 71 μL of water. The sample holders
(Supporting Information, Figure S2) were centered on the stir
plate, and glass coated stir bars (5 mm ꢀ 6 mm) were added to
each well. The desired amount of each metal salt solution was
added quickly (in under one second) to each well to give the
desired metal loading and ratio of the two metals (e.g., 10 μL of
each metal salt solution would give a 50:50 ratio of the two
metals with a total metal loading of 1 mol %). Then 300 μL of
ethanol followed by 298 μL of 1.788 M aluminum-sec-butoxide
were quickly (under 1 s) added to each well via pipet. After
stirring for approximately 5 min, 218 μL of the ethanol/hydro-
chloric acid/water solution was quickly (under one second)
added. Each sample holder was covered with parafilm; the
parafilm was punctured several times above each well, and the
solution was allowed to age and dry overnight in laboratory
ambient conditions. After processing, each sample holder was
placed in a vial, which was capped with a septum (Supporting
Information, Figure S2).
Experimental and Methods
Materials. CuCl2 2H2O (99.999%-Cu), CrCl3 6H2O,
HAuCl4 xH2O (99.9985%-Au), RuCl3 xH2O (99.9%-Ru),
3
3
3
3
IrCl3 xH2O (99.9%-Ir), MnCl2 4H2O (99.999%-Mn), PdCl2
3
3
(99.9%-Pd), CoCl2 6H2O (99.999%-Co), Na2PtCl4 xH2O,
3
3
NiCl2 6H2O (99.999þ%-Ni), FeCl2 4H2O (99%), MoCl5
3
3
(anhydrous, 99.6%), RhCl3 xH2O (38-41% Rh), and 0.5%
3
Rh/Al2O3 (pellets) were purchased from Strem Chemicals and
were used without further purification. Aluminum-sec-butoxide
(95%), decahydronaphthaplene, cis þ trans (97%), and methyl-
cyclohexane (99þ%) were purchased from Alfa Aesar. Ethanol
(100%, anhydrous) was purchased from Commercial Alcohol.
Millipore water was used throughout. Hydrochloric acid (con-
centrated), and silica gel were purchased from EMD. Dichloro-
methane (ACS), isopropanol, toluene were purchased from
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Processing. All catalysts were calcined in air in a tube furnace
using the following program: heat to 65 °C (rate 1 °C/min), hold
at 65 °C for 30 min, heat to 250 °C (rate 1 °C /min), hold at
250 °C for 3 h, cool to 25 °C (rate 1 °C /min). Then the catalyst
was hydrogen annealed under a 5% H2/95% Ar atmosphere as
follows: heat to 300 °C (rate 5 °C/min), hold at 300 °C for 3 h,
cool down to 25 °C rapidly by opening the furnace.
Screening. A solution of 7.7 mL of isopropanol, 0.456 mL of
decahydronaphthalene (internal standard), and 0.308 mL of
toluene was prepared. A necessary number of lines on the
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Inorganic Chemistry, Vol. 49, No. 6, 2010 2709
Schlenk line were split with Y joints, and additional tubes were
attached. A syringe barrel was inserted into the end of each tube,
and a needle was attached to each syringe. As described earlier
(Catalyst screening synthesis of 1 mol % metal loading) each
sample holder containing the prepared catalyst is located in an
individual vial. The septum on each vial was pierced with a
needle that was attached to the Schlenk line (Supporting In-
formation, Figure S3). The vials underwent three of the follow-
ing cycles: the vials were placed under vacuum and then
backfilled with Ar. This was repeated with Ar and then with
H2. One milliliter of isopropanol was added to the bottom of
each vial (but not to the sample holder), to minimize evapora-
tion losses from the sample holder. Next, 0.77 mL of the
previously prepared solution was added to each sample holder.
The vials were stirred for 4 h under 1 atm H2 pressure and at
22 °C. After 4 h, each sample holder was removed from the vial.
As much of the reaction mixture as possible was removed from
the sample holder with a Pasteur pipet. The reaction mixture was
filtered through a different Pasteur pipet packed with a small
amount silica gel directly into a GC vial. The substrate and
products were diluted with 1.0 mL of dichloromethane. The
screening hydrogenation results obtained for Rh0.5Pt0.5/Al2O3
were performed in triplicate.
CS2 Poisoning Studies. The following CS2 poisoning studies
were done based on a modified procedure by Hornstein et al.
and were performed in triplicate.91 A 0.3893 g portion of the
Rh0.5Pt0.5/Al2O3 catalyst or 0.782 g of the 0.5% Rh/Al2O3
catalyst was weighed into a clean 20 mL beaker. A glass coated
stir bar was added to the beaker followed by 5.0 mL of
isopropanol. A 0.038 M solution of CS2 in isopropanol was
prepared fresh each day, and the required amount of the
solution was added to the beaker. Ratios of 0, 0.02, 0.04, 0.06,
and 0.08 mol CS2/mol total metal were used for the Rh0.5Pt0.5
/
Al2O3 catalyst, and ratios of 0, 0.01, 0.02, 0.03, and 0.04 mol
CS2/mol total metal were used for the commercial catalyst 0.5%
Rh/Al2O3. Then an additional 5.0 mL of isopropanol was added
to the beaker followed by 0.40 mL of toluene. Decahydro-
naphthalene was not added. The beaker was placed into the
bottom of the Parr reactor with the hydrogen inlet placed
directly in the reaction solution, and then the reactor was
assembled. The Parr reactor was sealed, and the solution was
stirred for 30 s at a stir speed of 7. Then the stirrer was turned off,
and hydrogen gas was gently flowed through the reactor for 30 s.
The reactor was then sealed and pressurized to 5 atm. The course
of the reaction was monitored via a Parr pressure controller
interfaced with a computer for 1 h. After 1 h, the reactor was
depressurized and disassembled.
General Batch Hydrogenation. The appropriate amount of
catalyst to give 3.8 ꢀ 10-5 mol of metal was added to a 25 mL 3
neck round-bottomed flask. A glass coated stir bar was added,
and the flask was capped with two septa and a gas adapter. The
flask was placed under vacuum and then backfilled with Ar; this
was repeated two more times with Ar. Next, 10.0 mL of
isopropanol was added. The flask was placed under vacuum
and then backfilled with H2 for a total of three times. A 0.59 mL
portion (3.8 ꢀ 10-3 mol) of decahydronaphthalene and 0.40 mL
(3.8 ꢀ 10-3 mol) of toluene were added by syringe, and the
reaction was stirred under ambient conditions (1 atm H2,
22 °C). The reaction was monitored every 15 min for the first
2 h, and then every hour for an additional 5 h via gas
chromatography (GC). To prepare samples for GC analysis,
0.4 mL samples of the reaction mixture were removed from the
flask with a syringe. The contents of the syringe were filtered
through a silica gel filter as previously described. The bulk
hydrogenation results obtained for Rh0.5Pt0.5/Al2O3 were per-
formed in triplicate.
Sample Preparation for XPS, TEM, and BET. XPS: The dried
sample was first finely grounded to reduce the particle size. The
ground powders were then placed into a die and pressed into a
pellet under high pressure. The pellet was then used for XPS
analysis. TEM (before hydrogenation): Raw samples were care-
fully ground with a mortar and pestle for 20 min. The average
particle size is less than 100 nm after ground. A small amount of
the ground powder was then mounted on a carbon-coated grid
for TEM analysis. TEM (after hydrogenation): After reaction,
the solvent was removed/vaporized under vacuum overnight.
The raw sample was then ground and sprayed on a carbon
coated TEM grid. Brunauer-Emmett-Teller (BET): Before
analysis, samples were degassed at 250 °C for 23 h under
vacuum.
Results and Discussion
The field of arene hydrogenation is well established in-
dustrially with a varietyof heterogeneouscatalystsbeing used
(e.g., CoMo, MoW, and NiMo supported on alumina),4,7,92
but relatively harsh conditions are required to achieve re-
quiredcatalyst activities. As a result, there has been interest in
developing catalysts that are active under milder conditions.
For example, Rh NPs stabilizedby copolymersdemonstrated
a TOF (turnover frequency) of 250 h-1 and a TTO (total
turnover) of 20 000 mol/mol Rh for benzene hydrogenation
at 40 bar H2 and 75 °C.30,31 Magnetically recoverable Rh NPs
were very active for benzene hydrogenation with a TOF of
825 h-1 at 6 atm H2 and 75 °C.32 Pillai reported a TOF of
6600 h-1 for benzene hydrogenation and 4950 h-1 for toluene
hydrogenation at 20 bar H2 and 40 °C using Rh NPs on
multiwalled carbon nanotubes.11 Ambient conditions (1 atm
H2, 22 °C), however, are obviously the most challenging for
arene hydrogenation, and there are very few examples in the
literature.18,19,27-29,72 For example, Roucoux showed that
unsupported Rh NPs were active for benzene and toluene
hydrogenation under ambient conditions (1 atm H2, 20 °C)
with TOFs of 57 h-1 and 53 h-1, respectively (TOF reported
as mol H2 per mol of Rh).27 To discover new leads toward
Batch Toluene Studies. The hydrogenation was performed as
specified in the procedure for general batch hydrogenation, with
varying amounts of toluene depending on the desired ratio of
catalyst to substrate.
Batch Temperature Studies. The hydrogenation was per-
formed as specified in the procedure for general batch hydro-
genation, except that if heating the reaction mixture above room
temperature, a reflux condenser was attached to the middle neck
of the round-bottom flask and a gas adapter was connected to
the top of the reflux condenser. The other two necks on the
round-bottom flask were sealed with septa. For cooling below
room temperature, an ice bath was used to achieve a tempera-
ture of 0 °C, and a bath consisting of dioxane and CO2 (s) was
used for a 10 °C bath.
Batch Pressure Studies. The quantities of catalyst, solvent,
and substrate were scaled up by a factor of 2 for the batch
pressure studies, and decahydronaphthalene was not added.
The catalyst was added to the stainless steel bottom of the
pressure reactor, and then isopropanol and toluene were se-
quentially added. The pressure reactor was assembled, and
hydrogen was gently flowed through the reactor for 30 s. Then
the reactor was sealed, pressurized to the desired pressure, and
the course of the reaction was monitored via a Parr pressure
controller interfaced with a computer. After the reaction was
complete, the reactor was disassembled, and a sample of the
reaction mixture was analyzed by gas chromatography as pre-
viously described.
(91) Hornstein, B. J.; Aiken, J. D.; Finke, R. G. Inorg. Chem. 2002, 41,
1625–1638.
(92) Cooper, B. H.; Donnis, B. B. L. Appl. Catal., A 1996, 137, 203–223.

2710 Inorganic Chemistry, Vol. 49, No. 6, 2010
Dehm et al.
Scheme 1. Scheme Depicting the Steps for Catalyst Synthesis
discovery of supported heterogeneous arene hydrogenation
catalysts that are active under ambient conditions, we applied
a parallel synthesis and screening approach.
Figure 1. 3D bar graphs (a, top down view) and (b, side view) of
screening results. The x- and y- axes represent the different transition
metals used, with the z- axis showing the percent hydrogenation after 4 h.
Supported NP catalysts have been made by a variety of
methods, with one of the more common based upon impreg-
nation of an existing oxide supports.18,20,64 Another ap-
proach involves a multistep synthesis utilizing preformed
stabilized NPs that are then absorbed to the support.64 In
contrast, the in situ one pot method used by Maier involves
synthesizing the NPs and the support simultaneously; the
metal precursors and the water-sensitive metal alkoxide are
mixed together and processed in one batch (Scheme 1).81
These steps were followed by hydrolysis and condensation of
the metal alkoxide to give the metal oxide in which the metal
precursor is encapsulated. The metal precursor is not reduced
to NPs until the hydrogen anneal step is performed, leading
to reduced metal NPs on a metal oxide support. The
advantage of this approach is that it is a one-pot method,
no stabilizer is required for the NPs to form, and it can be
performed in laboratory ambient conditions, further simpli-
fying the procedure.
Catalyst Synthesis and Screening. Metal chloride salts
were used as the NP precursors, and over 90 mono- and
bimetallic NP catalysts were synthesized and tested for
arene hydrogenation activity based on the reaction shown
at the top of Figure 1. Since the goal of this work was the
development of air stable, easily handled and synthesized
catalysts, all steps involved exposure to an open atmo-
sphere. Our initial screening apparatus utilized a multi-
well glass plate, as shown in Figure S1 in the Supporting
Information, but cross-contamination by the volatile
toluene precursor and methylcyclohexane product led
to spurious results; a system that isolates each vessel
was required in which each vessel has its own separate
gas supply (Figures S2 and S3, Supporting Information).
To allow for easy identification of the active catalysts and
simple interpretation of the results, a visualization ap-
proach of the data was used, with the results shown in
Figure 1. The reactions were commenced and arrested
after 4 h to identify active catalyst combinations.
As can be seen from the side view of the bar graphs
shown in Figure 1b, there are several catalysts that are
active, and a large number of inactive combinations
under the screening conditions. From the top down view
of the 3D bar graph (Figure 1a), it can be easily identified
that bimetallic catalysts containing Rh were the most
consistent for activity, with the exception of the RhCu
combination. The deactivation of a catalytically active
metal upon alloying with Cu has been observed before
with a PtCu bimetallic catalyst, and was attributed to an
Figure 2. Bar graphs with varying ratios of metals, metal loading held
constant at 1 mol %. (a) IrRh/Al2O3, (b) RhPt/Al2O3, (c) IrPt/Al2O3,
(d) RuPt/Al2O3. The percent hydrogenation was measured after 4 h.
enrichment of catalytically inactive copper on the surface
of the NPs.93 From the screening results, the most active
catalysts were identified to be Rh0.5Pt0.5/Al2O3, Ir0.5Pt0.5
/
Al2O3, Ru0.5Pt0.5/Al2O3, Ir0.5Rh0.5/Al2O3, and Rh1/Al2O3
in order of highest activity to lowest activity, and the
results obtained for Rh0.5Pt0.5/Al2O3 were performed in
triplicate. It is of note that the monometallic Ru1/Al2O3,
Pt1/Al2O3 and Ir1/Al2O3 catalysts were inactive for the
hydrogenation of toluene under these conditions, but
upon alloying with Rh or Pt, the bimetallic catalysts
exhibited a higher activity than either of their parent
metals.
From the screening results, the four most active bime-
tallic catalysts (Rh0.5Pt0.5/Al2O3, Ir0.5Pt0.5/Al2O3, Ru0.5Pt0.5/
Al2O3, and Ir0.5Rh0.5/Al2O3) were then screened again
using varying ratios of the two metals in 10% increments
while holding the metal loading constant at 1 mol %.
These results are shown in Figure 2. In the case of IrRh
(93) Hoover, N. N.; Auten, B. J.; Chandler, B. D. J. Phys. Chem. B 2006,
110, 8606–8612.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2711
Table 1. Batch Toluene Hydrogenation Results from First Generation Screen-
ing.a
entry
catalystb
Rh1
Ir0.5Rh0.5
Ir0.5Pt0.5
obs. rate [h-1
]
% conv. (2 h) % conv. (7 h)
1
2
3
4
5
6
7
8
9
7.7
17.8
22.8
27.2
23.1
51.3
26.2
45.4
14.4
12.2
58
36
47
52
90
54
85.3
46.0
42.9
11.1
12.8
12.4
24.9
14.6
Ru0.5Pt0.5
Rh0.5Pt0.5
Rh0.5Pt0.5
c
d
Rh0.25Pt0.25 27.1
Rh1Pt1
Rh2.5Pt2.5
6.2
5.0
a Reaction conditions: 3.8 ꢀ 10-5 mol metal, 10.0 mL of isopropanol
substrate/cat. = 100:1 = 3.8 ꢀ 10-3 mol toluene, 22 °C, 1 atm H2. b All
catalysts were supported on Al2O3, with the subscripts indicating the
mol % of the metals. c Observed rate based on results obtained in
triplicate. d Ethanol used as the solvent instead of isopropanol.
(Figure 2a), the more active catalysts were those that are
richer in Rh, whereas with RhPt (Figure 2b) the more
active catalysts were those that were richer in Pt. In
contrast to these trends, with IrPt and RuPt (Figure 2c
and 2d), the most active catalysts were those that have
approximately equal amounts of the two metals.
The five most active catalysts (Rh0.5Pt0.5/Al2O3,
Ir0.5Pt0.5/Al2O3, Ru0.5Pt0.5/Al2O3, Ir0.5Rh0.5/Al2O3, and
Rh1/Al2O3) were synthesized and tested in bulk to con-
firm their activity with the results shown in Table 1.
Toluene hydrogenation reactions were performed at at-
mospheric pressure and temperature in laboratory batch
reactors. The progress of the reactions was monitored
through gas chromatography in which no partially hy-
drogenated intermediates were detected, only the fully
hydrogenated product of methylcyclohexane, which was
also confirmed for a representative reaction by GC-MS.
The observed rate (obs. rate) relative to the methylcyclo-
hexane hydrogenation product was calculated during the
first 2 h of hydrogenation results, where
mol methylcyclohexane=mol metal in catalyst
obs:rate ¼
timeðhÞ
As can be seen from Table 1, entries 1-5, Rh0.5Pt0.5
/
Al2O3 was confirmed to be the most active catalyst with
an observed rate of 24.9 ( 2.8 h-1 based on experiments
performed in triplicate. The next most active catalysts
Figure 3. (a) Rate as a function of temperature for the hydrogenation of
toluene. (b) Arrheniusplot, withtheslopeofthetrend lineequaling-3.65.
(c) Observed rate as a function of the ratio of toluene to metal. Lines
within the plots are drawn merely as a visual aide.
were determined to be Ir0.5Pt0.5/Al2O3 and Ru0.5Pt0.5
/
have been used as CO oxidation catalysts,96,97 and the
RhPt combination has been used as an electrocatalyst for
the dehydrogenative oxidation of cyclohexane to ben-
zene.98
Discussion of Catalytic Activity. Since the RhPt bime-
tallic combination led to the highest observed rate, this
catalyst system was studied in further detail. Toluene
hydrogenation reactions were performed on the alumina
supported RhPt bimetallic catalyst in bulk in a laboratory
batch reactor. Observed rates were measured during the
first 2 h of the hydrogenation reaction and normalized
against the number of moles of metal in the catalyst. A
number of parameters were varied, including metal loading,
Al2O3 followed by Ir0.5Rh0.5/Al2O3. The trends in bulk
activity correspond well with the trends observed for the
screening results (Supporting Information, Figure S4),
validating the screening results. While none of these
bimetallic catalysts have been previously reported as
arene hydrogenation catalysts with the exception of RuPt
(Midgley reported Ru5Pt1 and Ru10Pt2 to be successful
benzene hydrogenation catalysts at 80 °C and 20 bar
H2 pressure53), it is of note that RuPt NPs have been
previously used as a catalyst for preferential CO oxida-
tion in the presence of hydrogen feeds,94,95 RhPt NPs
(94) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Nat.
Mater. 2008, 7, 333–338.
(95) Alayoglu, S.; Eichhorn, B. J. Am. Chem. Soc. 2008, 130, 17479–
17486.
(96) Abdelsayed, V.; Aljarash, A.; El-Shall, M. S.; Al Othman, Z. A.;
Alghamdi, A. H. Chem. Mater. 2009, 21, 2825–2834.
(97) Park, J. Y.; Zhang, Y.; Grass, M.; Zhang, T.; Somorjai, G. A. Nano
Lett. 2008, 8, 673–677.
(98) Kim, H. J.; Choi, S. M.; Nam, S. H.; Seo, M. H.; Kim, W. B. Appl.
Catal., A 2009, 352, 145–151.

2712 Inorganic Chemistry, Vol. 49, No. 6, 2010
Dehm et al.
Table 2. Summary of the CS2 Poisoning Results and Catalytic Activities of the
Prepared Rh0.5Pt0.5/Al2O3 and Commercial 0.5% Rh/Al2O3 Catalystsa
Rh0.5Pt0.5/Al2O3
0.5% Rh/Al2O3
rateb
TOF
0.76 ( 0.16 psi/h
0.47 ( 0.13 psi/h
27.2 h-1
0.046
43.8 h-1
0.081
108 h-1
1
mol CS2/mol total metalc
TOF (corrected)d
rel. TOF
118 h-1
1.09
a Catalytic activities measured at 5 atm H2 pressure, 295 K. b Values
based on 3.8 ꢀ 10-5 mol of total metal. c Ratio of mol CS2/mol total
metal required to deactivate catalyst. Determined from results shown in
Supporting Information, Figure S6. d TOF is corrected for active Rh and
Pt atoms determined by CS2 poisoning and using a 1/5 postion/metal
stoichiometry ratio, as previously described by Hornstein et al.91
temperature, amount of substrate, and hydrogen pres-
sure, to determine the effect these variables would have on
the observed rates of the RhPt catalyst. A CS2 poisoning
study was also performed on the RhPt catalyst to deter-
mine the number of catalytically active sites in the cata-
lyst.91 The hydrogenation of toluene was performed using
ethanol as solvent (Table 1, entry 6), and a decrease in the
observed rate of the catalyst was observed; therefore
isopropanol was used as the solvent for the remainder
of this study. The metal loading of the RhPt catalyst was
varied between 0.5% and 5% while holding the ratios of
Rh and Pt constant (Table 1, entries 7-9). It was found
that the observed rate increased slightly as the loading
was decreased to 0.5% but with metal loadings of 2% and
5%, the observed rate greatly decreased.
The temperature of the hydrogenation reaction was
increased to determine the range of temperatures in which
the catalyst was active. Initially, the temperature of the
hydrogenation reaction was varied in 20 degree incre-
ments from 0 to 80 °C, and the results are shown in Figure 3a.
The observed rate of the reaction increased somewhat
linearly with respect to temperature, with the maximum
rate occurring at 60 °C. At the highest temperature
studied (80 °C), there was a substantial decrease in
activity, suggesting that deactivation of the catalyst was
occurring. To allow for comparison among other known
arene hydrogenation catalysts, the hydrogenation of
toluene was examined, in triplicate, between 0 and 40 °C
to calculate the activation energy. The initial rates were
measured during the first 30 min of the toluene hydro-
genation reaction. The results were plotted in an Arrhe-
nius plot (Figure 3b), and the activation energy was
calculated to be 30.4 kJ/mol. This activation energy is
moderate when compared to those previously reported
for arene hydrogenation. Dupont et al., for instance,
reported an activation energy of 42.0 kJ/mol for the
hydrogenation of toluene with Ru NPs1 and Somorjai
et al. reported activation energies of 34.7 and 45.6 kJ/mol
for benzene hydrogenation with cuboctahedral and cubic
Pt NPs, respectively.2
The molar ratio of toluene to metal in the catalyst was
varied at ratios of 50, 200, 500, and 1000, with the results
shown in Figure 3c. The highest observed rates were
obtained for the ratios of 200 and 500. When the ratio
was further increased to 1000, there was a substantial
decrease in activity, suggestive of substrate inhibition.
Preliminary pressure studies were also done, and showed
a relatively linear increase in reaction rate versus hydro-
gen pressure. (Supporting Information, Figure S5).
Figure 4. TEM images and particle size histograms of NP catalysts:
(a) Rh0.5Pt0.5/Al2O3, (b) Ir0.5Pt0.5/Al2O3, (c) Ir0.5Rh0.5/Al2O3, (d) Ru0.5Pt0.5/
Al2O3, (e) 0.5% Rh/Al2O3.
To determine the number of catalytically active sites
present, and to allow for more accurate comparison of the
activities of different catalysts, a series of CS2 poisoning
experiments were performed on the Rh0.5Pt0.5/Al2O3 and
0.5% Rh/Al2O3 catalysts in triplicate.91 Known amounts

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2713
Figure 5. X-ray photoelectron spectra survey scans for Rh0.5Pt0.5
/
Al2O3. The spectra indicated in black (a) corresponds to the catalyst
before the toluene hydrogenation reaction, and the spectra in red
(b) corresponds to the catalyst after the toluene hydrogenation reaction.
of CS2 were added to the toluene hydrogenation reaction,
and the activity of the catalysts was measured. The rates
of the unpoisoned Rh0.5Pt0.5/Al2O3 and 0.5% Rh/Al2O3
catalysts were measured at 5 atm H2 and 295 K, and the
results are shown in Table 2. Using these rates, the TOFs
were calculated based on the molar amount of metal
used, which was the same for the two different catalysts.
As already shown in Table 2, the Rh0.5Pt0.5/Al2O3 cata-
lyst is more active, with a TOF of 43.8 h-1, than the
Figure 6. High-resolution XPS of the Rh 3d peak in Rh0.5Pt0.5/Al2O3.
The spectra indicated in black (a) is before the toluene hydrogenation
reaction and the spectra indicated in red (b) is after the toluene hydro-
genation reaction. The binding energy for Rh 3d before and after reaction
are 306.7 and 307.1 eV, respectively.
0.5% Rh/Al2O3 catalyst which has a TOF of 27.2 h-1
.
Upon performing the poisoning studies, the amount of
CS2 required to completely deactivate each catalyst
was calculated to be 0.081 mol CS2/mol total metal for
the Rh0.5Pt0.5/Al2O3 catalyst and 0.046 mol CS2/mol
total metal for the 0.5% Rh/Al2O3 catalyst (Support-
ing Information, Figure S6). The higher value for the
Rh0.5Pt0.5/Al2O3 catalyst suggests that there are more
catalytically active sites present in this bimetallic catalyst
than in the commercial catalyst. Hornstein et al. assumed
a 1/5 poison to metal-atom stoichiometry ratio to calcu-
late corrected TOF values for each catalyst.91 On the
basis of this assumption, the corrected TOFs for the
Rh0.5Pt0.5/Al2O3 and the 0.5% Rh/Al2O3 catalysts are
108 h-1 and 118 h-1, respectively. Although these values
for the corrected TOFs are similar, the need for a higher
quantity of catalyst poison (CS2) for the bimetallic
catalysts suggests that there are more active (but less
reactive) sites than in the commercial 0.5% Rh/Al2O3
catalyst.
Characterization. Since the synthesis is based upon a
one pot in situ production of both the support and the
NPs simultaneously, little is known about the morphol-
ogy or structure of the resulting catalyst. From the TEM
image of Rh0.5Pt0.5/Al2O3 catalyst (Figure 4a) the size of
the NPs was measured to be 3.1 ( 1.0 nm. When TEM
images were obtained upon the completion of the hydro-
genation reaction using the Rh0.5Pt0.5/Al2O3 catalyst,
the size of the NPs was measured to be 3.3 ( 1.1 nm,
indicating no significant change in size or morphology of
the NPs during the course of the reaction (Supporting
Information, Figure S7). The size of the NPs in the
Ir0.5Pt0.5/Al2O3 catalyst was measured to be 2.9 ( 0.9
nm (Figure 4b), 3.8 ( 1.2 nm for the Ir05Rh0.5/Al2O3
catalyst (Figure 4c), 4.2 ( 1.5 nm for the Ru0.5Pt0.5/Al2O3
Figure 7. High-resolution XPS of Pt in Rh0.5Pt0.5/Al2O3. The spectra
indicated in (a) are before the toluene hydrogenation reaction, and the
spectra indicated in (b) are after the toluene hydrogenation reaction. The
BE for Pt before and after reaction are 71.4 and 71.2 eV, respectively.
catalyst (Figure 4d), and 5.2 ( 1.6 nm for the commercial
0.5% Rh/Al2O3 catalyst (Figure 4e).
XPS analysis of the Rh0.5Pt0.5Al2O3 catalyst was per-
formed to determine the oxidation states of the two
metals present in the catalyst. The survey scan shown in
Figure 5 shows the XPS spectra before (spectra shown in
black) and after (spectra shown in red) the toluene
hydrogenation reaction. The two spectra are very similar,
and the Rh 3d, Al 2s, Al 2p, and Pt 4f peaks can be clearly
identified in both, though the Al 2p and Pt 4f peaks
overlap.
From the high-resolution spectra of Rh in Rh0.5Pt0.5
/
Al2O3 (Figure 6) the binding energy (BE) of Rh was
measured to be 306.7 eV before the reaction and 307.1
eV after the reaction. From the high-resolution spectra of
Pt in Rh0.5Pt0.5/Al2O3 (Figure 7), a curve-fitting program
was used to obtain information about the Pt 4f peak since
it overlaps with the Al 2p peak. The BE of Pt was

2714 Inorganic Chemistry, Vol. 49, No. 6, 2010
Dehm et al.
determined to be 71.4 eV before the hydrogenation and
71.2 eV after the hydrogenation. By considering the BE of
the Rh and Pt peaks before and after the hydrogenation
reactions, it can be seen that there was no substantial
deviation in the BE before and after the hydrogenation,
suggesting that the oxidation state of the metal does not
change substantially.
Conclusions
Through a simple screening approach, four active bime-
tallic catalysts (RhPt, RuPt, IrPt, and IrRh) were identified.
Through confirmation of the results inbulk, Rh0.5Pt0.5/Al2O3
was found to be the most active catalyst, which was then
tested under a variety of parameters, and the activation
energy was measured to be 30.4 kJ/mol. Through the use of
standard materials characterization techniques, all of the
bimetallic catalysts were determined to consist of small, zero
oxidation state NPs that were well dispersed throughout
the alumina supports. Future work on this project will
involve screening trimetallic combinations for catalytic acti-
vity, and screening for activity on mixed metal oxide supports
and in the presence of sulfur and nitrogen containing com-
pounds.
XPS spectra were also obtained for the 0.5% Rh/
Al2O3, Ru0.5Pt0.5/Al2O3, Ir0.5Pt0.5/Al2O3, and Ir0.5Rh0.5
/
Al2O3 catalysts. For the commercial catalyst, 0.5% Rh/
Al2O3, the high resolution XPS spectra showed a BE of
308.8 eV for Rh (Supporting Information, Figure S9),
suggesting that the Rh was oxidized to Rh(III). For
Ru0.5Pt0.5/Al2O3 (Supporting Information, Figures S10
and S11), the BE for Ru was 280.1 and 71.2 eV for Pt,
indicating that both metals are in the zero oxidation state.
From the high-resolution spectra for Ir0.5Pt0.5/Al2O3
(Supporting Information, Figures S12 and S13) the BE
for Ir was 60.7 and 71.4 eV for Pt, indicating that both
Acknowledgment. The Alberta Ingenuity-Imperial Oil
Centre for Oil Sands Innovation (COSI), NINT-NRC,
the University of Alberta and CRC programs are thanked
for their generous financial support. Dr. Mike Xia of the
NINT Chemical Analysis Lab is thanked for performing
the GC-MS analysis and obtaining the BET measure-
ments. The staff of the Department of Chemistry Glass
Shop are thanked for making the sample holders. Jian
Chan and Peng Li from the NINT Electron Microscopy
Lab are thanked for obtaining the TEM images. Shihong
Xu from the Alberta Centre for Surface Engineering and
Science (ACSES) is thanked for obtaining the XPS data.
metals were in the zero oxidation states. For Ir0.5Rh0.5
/
Al2O3 (Supporting Information, Figures S14 and S15),
the BE for Ir and Rh were 60.4 and 306.7 eV, respectively,
corresponding to the zero oxidation states for both
metals.
Since acid-catalyzed sol-gel syntheses are known to
give high-surface area metal oxides,99 BET measurements
were performed to measure the surface area of the blank
support and the catalyst. The surface area of the blank
support was measured to be 434 m2/g, which is in the
range of surface areas reported in the literature for
alumina.100,101 The surface area of the Rh0.5Pt0.5/Al2O3
catalyst was measured to be 557 m2/g (Supporting Infor-
mation, Figure S16).
Supporting Information Available: Images of sample holders
and set up; plot of comparison of trends observed through
screening with those observed in batch; plot of observed rate
versus moles of CS2/mols of total metal for the hydrogenation of
toluene by Rh0.5Pt0.5/Al2O3 and 0.5% Rh/Al2O3; TEM image
and histogram of Rh0.5Pt0.5/Al2O3 NPs after the hydrogenation
of toluene; XPS spectra of Ir0.5Pt0.5/Al2O3, Ru0.5Pt0.5/Al2O3,
Ir0.5Rh0.5/Al2O3, and 0.5% Rh/Al2O3 catalysts; the 5-point
BET surface area plot. This material is available free of charge
via the Internet at http://pubs.acs.org.
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